In recent trends in evaluating the machining capabilities of a lathe, it is important not only to improve the cutting ability, the issue of shortening the automatic tool changing time of a turret is also an important factor to be considered in lathe design. A turret with high speed and high precision is the trend of CNC lathe design. The currently-used positioning mechanism of a turret index, as shown in FIG. 1, uses motor 10 and reducing gear set 11 as a transmission mechanism to drive output shaft 12 to perform the index rotation; Further the positioning pin 14 is pushed by electromagnetic actuator 13 to work out the preliminary positioning. Then the coupler 16 is meshed through the cam meshing mechanism 15 to attain precise positioning. The use of a turret index positioning pin to stop the rotational action caused by rapid large loading, will create an inertial bombardmental effect with loud noise and it can very easily damage the positioning pin 14 and cause failure of the turret index device.
Shown in FIG. 2, "Positioning Mechanism of Turret Index," is the invention described "Utility Model No. 85648 Patent" of the Republic of China. It transmits a dynamic input shaft 21 with hydraulic motor 20. The input shaft has a parallel conjugate cam 22 and a cylindrical cam 23 set up and fixed on it. There are a rotational disk 25 and a push ring 28 set up and fixed to its output shaft 24, moreover, it performs precise positioning with a three-piece toothed coupler 29, 30, 31, which are all in annular shapes wherein the first piece 29 is fixed to the push ring 28, the second piece 30 is fixed to the housing 32 and the third piece 31 is fixed to the output shaft 24 and its third piece 31 is located in the inner circle of its second piece 30. When hydraulic motor 20 drives both the conjugate cam 22 and cylinder cam 23 to rotate, it can cause the rocker arm 27 to move to the left or right and the rocker arm 27 can push the push ring 28 to move to the left or right so as to make the combining and separating operation among the first, second and third pieces of the three pieced toothed couple. As the three pieced toothed coupler is in separated condition among the first, second and third pieces, the conjugate cam 22 will pluck the roller 26 on the follower disk 25 and cause the follower disk to rotate to attain the object of making the output shaft 24 to perform index rotation. As the three-piece toothed coupler is in meshing condition among the first, second and third pieces, it can attain precise positioning.
When the tool on the output shaft (not shown in the Figures) performs machining on the workpieces, it will generate torsion. In order to prevent the first piece of the toothed coupler from being retreated and separated, a strong disk spring 33 is set up at one end of the cylindrical cam 23 in the mechanism as shown in FIG. 2. As the output shaft 24 is subjected to torsion while its tool is performing machining and when the first piece of the toothed coupler can not be retreated and separated, this torsional force can only cause the output shaft to generate a very small prop-up. This will affect the precision of machining. Also the operators need to undertake multiple indexing operations, that is, the toothed coupler requires multiple teeth's meshing bombardment whenever the operators intend to change the remote tools, thus, it not only can generate multiple bombarding noise, but also take time on operation. Furthermore, the design and machining efforts required for the cylindrical cam, slider and rocker arm are not easy, considering the cost, which will result in an increase in cost.
There is a U.S. Pat. No. 5,271,134 "Tool Turret with Axial Reactive Force", as shown in FIG. 3, to resolve the rigidity problem of the output shaft. The first piece of the three-piece toothed coupler in the precision positioning mechanism is to be activated by use of hydraulic system. As the oil flows into the oil chamber, the hydraulic acting force F1 applies on the first piece 34 of the toothed coupler which tightly presses the second and third pieces of the toothed coupler; in the same time, it makes use of the reacting force FR to apply on the push block 40, connecting ring 41 and gear rotating axis 37 to prevent the spindle from propping outward and losing its rigidity when the toothed coupler is meshing. But such reacting force FR is equal to the tight-pressed force F1 only and is unable to keep its rigidity. Hence a push block combination 38 is also designed, filling the oil chamber with hydraulic pressure, to generate hydraulic pushing force F2 so as to assist the reacting force FR possessing sufficient tightening force in axial direction to ensure rigidity, thereby, it becomes very complicated in the overall design.
The inventors, therefore, devoted themselves to study and consideration of the actual CNC lathe, in view of the existing shortcomings which includes inabilities to change tools at high speed and complicated design, of the custom-used turret index positioning mechanism, and eventually developed an improved precision positioning mechanism which not only can simplify the mechanism but also can change tools at high speed as well as keep the rigidity of the output shaft.